AC Speed Control

[georgebarr]

Is there a way to create a circuit that will control the speed of an AC motor (like a rotor motor)? I am thinking of using a PIC or BASIC Stamp style circuit that will interface with the computers serial port. The computer should be able to send controls to tell the AC motor what speed it should go at. It will also need an encoder to sense position and speed. I have seen a DC version of this but no AC version.

[HuFlungDung]

Hi George,

AFAIK, AC motors are totally frequency dependent. That would be the 60 cycle standard in North America, or the 50 cycle standard in Europe.

Hence, the evolution of the variable frequency drive for AC motors.

The operating principle of AC and DC motors differ. The DC motor has a commutated rotor, whereas the typical AC motor does not. The DC motor will switch its own windings sequentially depending on the position of the brushes and commutater, but it operates on a zero frequency current.

The AC motor does not switch this way, but relies on the "external clock" imposed by the power grid frequency: thus it's magnetic poles switch polarity at a given rate as the current rises and falls, and this determines the pulse rate of the motor. There is a fair degree of "slippage" in any AC motor because they should rotate at 1800rpm theoretically, but typically only achieve 1725 to 1750.

Good quality variable frequency drives do rely on encoder feedback to maintain a given rpm. This is actually more accurate than running the motor "off the powerline", where you never know what speed you are getting, depending on the load.

[georgebarr]

HuFlungDung,

Do you know any www sites of some cheap but usable VFD's? Does all VFD's have the feedback feature? Also, please list some manufacturers.

Thanks,

[Al_The_Man]

George, You did not indicate what type of AC motor you want to control, is it a standard 60 hz single phase or a three phase motor, if so there are some deals that have come up in the past on the Mitsubishi site on ebay, they are getting rid of last years models vector drives. most of these up to around 5hp can drive a three phase motor from single phase (or 3 ph) supply. You just connect 230 single ph on to the R&S terminals of the R,S&T input. They do not need any kind of feedback, but they are for speed control only, not positioning, you would need the encoder feedback option card.
They will accept +-10v command signal. If it a small motor you need and want positioning look at a AC servo motor & amp. It all depends on your application.
Al

[HuFlungDung]

George,

Check out Ebay, like Al suggested. The VFDrive type that I am referring to would be for spindle motors, not for positioning applications.

I have had good results using Baldor as a supplier for both Vectorless and Vector drives.

Vectorless denotes a VFD that tries to hold rpm by sensing the current load, without encoder feedback. I am not sure of the accuracy range, but I do know that low speed torque is practically absent with this type of drive.

The Baldor Vector drive uses encoder feedback. This gives excellent speed control, but not position control. These are ideal for tapping and drilling operations done at low speeds on a milling machine, because the drive will strive to maintain the motor at the commanded speed, which means excellent torque control.

Either type of drive will permit supplying high frequencies to your motor, which can give you "overdrive" without belt changes, etc. Most single speed motors designed to run at 1800 rpm can be run up to 3600 rpm or faster, however, the rotor may need to be balanced, and the motor fan replaced. Old motors may short out as the high frequency pulses pound away at the old insulation on the windings.

AC servo motors are also available, but are of special internal construction (Hall sensors or something) which allow the motor to be commutated and positioned accurately via software.

[georgebarr]

I plan to use a porter cable router that uses a 110V at 60Hz (from the home AC outlet). I am not sure how many phases it uses. I do want to perform tapping and drilling so I need the encoder feedback. I will look into the Baldor Vector drive. I noticed that most of the VFD's require a 230V input/output. I need a 110 input/output. Can someone tell me of the porter cable router found at Home Depot is 1 or 3 phases?

[balsaman]

Routers don't need VFD's. They are a universal motor and can be controlled with a cheap and readily available router speed control. varying the voltage varies the speed on this type of motor. You won't be tapping with a router tho. They are too high RPM for tapping. Low torque too. Not great for drilling either, but acceptable in wood.

I would say you need a different type of spindle to do this type of work.

Eric

[HuFlungDung]

Yes, Eric is right about the universal motor.

If you needed to, you could build a secondary spindle for your machine, powered by a regular 220v 3 phase motor. As Al pointed out, you can run single phase power into a VFD, and get 3 phase out, but the VFdrive is derated by 50%. That means you would need VFD twice as large as your motor nameplate hp.

One additional feature of the VFDs that is nice is the dynamic braking, with programmable accel/decel. This eliminates the need for a motor brake, and yet prevents excessive spindle run-on.

In this same vein, when you command a Vector drive with encoder feedback to stop the motor, it stops it and holds it at 0 rpm. A Vectorless drive will brake the motor, but the holding torque is nil once it has stopped. So if you were relying on a motor brake to perform a tool change or something, this could be an important feature to know about.